1. Field of the Invention
This invention relates generally to well production equipment, and in particular to a well production system employing flexible tubing.
2. Description of the Prior Art
Various types of artificial lifts are used to bring liquids to the surface of the earth when the pressure of the liquid-bearing reservoir is insufficient to produce the formation fluids by natural means. The pumping motion of the artificial lift may originate at the surface of the ground, or below, as a result of the application of electrical or fluid power to a subsurface pump. In the usual surface powered rig, a reciprocating pump element at the bottom of the well is mechanically actuated by a walking beam pivotally mounted on a Sampson post and connected at one end to a sucker rod string and at the other end to a prime mover which supplies power through a Pitman gear for producing reciprocating motion.
Generally, in the surface powered rig, the prime mover consists of an internal combustion engine or electrical motor. The cost of this prime mover, as well as its operation and maintenance is, in many instances, a significant economic factor in the production of liquids from subterranean liquid-bearing reservoirs. The sucker rod is characterized by a short, fast stroke resulting in low pump efficiency, high power consumption and low recovery rates.
An additional limitation of surface-powered rigs which operate a sucker rod through rigid production tubing is that, for practical purposes, the weight of the sucker rod for wells having a producing formation at about 10,000 feet or deeper is excessive, resulting in stretching and early failure of the rods. Moreover, the sucker rod assembly is subjected to severe wear in slant-hole or crooked-hole wells. In slant-hole wells of the type typically drilled offshore, the sucker rod is subjected to severe frictional wear because of the slant of the hole, and is therefore subject to early failure and requires frequent replacement. Frictional wear is also a serious problem in crooked holes in which the well bore follows a helical path. Because of the difficulty of drilling a vertical well at other than shallow depth, effective use of the sucker rod pumping assembly is substantially limited to shallow wells.
For the foregoing reasons, there has been considerable interest in improving pumping systems in which the motive force is provided by electrical or pneumatic/hydraulic power applied to a subsurface pump, thereby eliminating the sucker rods and affording precise control of the pumping action.
Submersible pumps of the type in which the motive force is derived from electrical, hydraulic or pneumatic power applied downhole represent a significant improvement over the reciprocating sucker rod approach. According to conventional practice, the submersible pump which is driven downhole by the electrical/hydraulic or pneumatic driving means is supported at the end of a long string of rigid production tubing joints which are coupled to each other by pin and box connections. Power conductors, signal conductors and hydraulic/pneumatic conduits are bundled together in an external umbilical cable assembly supported by the rigid production tubing joints, and are coupled to the downhole pump.
The rigid tubing installations must be worked over from time to time due to either faulty downhole equipment or to some other unusual or adverse well condition. For example, if the production string is damaged or leaking, it may be necessary to pull the production tubing and umbilical from the casing and replace the damaged section.
When such service operations become necessary, a portable installation called a work-over rig is brought to the well site and set up. Generally, these rigs consist of a heavy derrick or mast which support work pulleys or block-and-tackle arrangements which are operable to pull the pipe string from the well. The work-over rigs are heavy and difficult to erect and must be capable of lifting the substantial load imposed by the rigid production pipe string.
An overriding concern in the operation of a producing well is to get the necessary equipment into and out of the well as rapidly and safely as economically possible. The efficiency of the pipe handling operation depends upon such factors as the running speed of the hoist rig, the time required to make up or break a tool joint during stabbing operations, the time required to mechanically couple and decouple the hoist rig and the pipe string, and the time required to transport the length of pipe from the pipe string to a storage station during recovery operations and to transport a length of pipe from the storage station to the pipe string during launching operations.
As the length of the rigid pipe string increases to reach the producing formation of deep wells, the cost of providing and operating rigid pipestring equipment becomes a significant factor. Moreover, the weight of rigid pipestrings and handling equipment becomes a limiting factor in offshore installations.